1. Field of the Invention
The present invention relates to a plated member, a plated terminal for connector, a method for producing a plated member and a method for producing a plated terminal for connector and more particularly to a plated member and a plated terminal for connector containing silver and tin in a plating layer and methods for producing them.
2. Description of the Related Art
In recent years, high-output motors have been used in hybrid vehicles, electric vehicles and the like. Since a large current flows in a connector terminal such as a terminal for high-output motor to which a large current is applied, the amount of heat generation in a terminal portion increases. Further, since the terminal becomes larger in conformity with a current capacity, a larger insertion force is required and damage on a terminal surface during insertion becomes larger. The terminal is inserted and withdrawn a large number of times for maintenance. Connector terminals for large current of this type are required to be heat resistant and abrasion resistant.
Conventionally, connector terminals formed by plating a surface of a base material such as copper or copper alloy such as with tin have been generally used as connector terminals for connecting electric components and the like of automotive vehicles. However, conventional tin plated terminals are insufficient in heat resistance when being used with such a large current. Accordingly, instead of tin plated terminals, silver plated terminals are used as connector terminals used with a large current. Silver has a low electrical resistance value, suppresses a temperature increase when a current is applied, has a high melting point and provides high heat resistance. Further, silver plating has very high corrosion resistance.
However, since copper particles easily diffuse in a silver plating layer, there is a problem that copper components reach a silver plating surface and the oxidized copper components increase resistance in the case of applying silver plating to a surface of a terminal base material made of copper of copper alloy. Further, silver has a property that crystal grains are easily coarsened through recrystallization. If a silver plated terminal is used under a high-temperature environment, hardness is reduced due to the growth of crystal grains. This causes a problem of increasing a terminal insertion force and increasing a friction coefficient.
Accordingly, Japanese Unexamined Patent Publication No. 2009-79250 discloses a copper or copper alloy member in which nickel underplating is applied to a surface of a copper or copper alloy member, a soft silver layer or silver alloy layer with a limited antimony concentration is formed on the nickel under plating, and a hard silver alloy layer containing antimony is formed as an outermost layer on the soft silver layer or silver alloy layer for the purpose of combining hardness and heat resistance.
On the other hand, an attempt to form an alloy of silver and tin and contribute to an improvement of heat resistance and a friction coefficient reduction has also been reported. Publication of Japanese Patent No. 4372835 discloses an electrically conductive member in which an Ni-based underlayer is formed on a copper-based base plate and a Cu—Sn intermetallic compound layer is formed on an Sn—Ag coating layer.
A plurality of plating structures including a silver-tin alloy layer and their producing methods are known although they have purposes different from an improvement of heat resistance and a friction coefficient reduction. For example, Japanese Unexamined Patent Publication No. 2010-138452 discloses to form an Ag3Sn alloy layer by, after forming an Sn layer on a surface of a base material made of Cu or Cu alloy by electroplating, forming an Ag nanoparticle coating layer on this Sn layer by a wet film forming method. Further, Japanese Unexamined Patent Publication No. 2008-50695 discloses to form an Ag—Sn intermetallic compound by forming an Ag thin film on an Sn thin film. Here, an outermost surface is made of the Ag—Sn intermetallic compound.
In the plating layer structure described in each of the above patent literatures, abrasion resistance by the friction coefficient reduction and heat resistance by the suppression of a resistance increase when being left at high temperature are not combined at a sufficient high level as a terminal for large current.
In Japanese Unexamined Patent Publication No. 2009-79250, a main plating layer is the soft silver layer and sufficient abrasion resistance cannot be obtained even if the hard silver layer is formed thereon. Also in the structures of patent literatures 2 to 4, the Ag—Sn alloy is formed on the outermost layer and the abrasion resistance of this alloy is not very high.
Further, in the plating layer of Japanese Unexamined Patent Publication No. 2009-79250, copper atoms diffuse into the silver plating layer due to exposure to a heating environment for a long time and are oxidized on the outermost surface of the plating layer, thereby increasing a surface resistance value. Even if the Ni underplating is formed, it is insufficient to prevent the diffusion of copper atoms from the base material. Further, antimony atoms contained in the hard silver layer also diffuse into the outermost surface and are oxidized to increase the surface resistance value. On the other hand, in Japanese Patent No. 4372835, Japanese Unexamined Patent Publication No. 2010-138452 and Japanese Unexamined Patent Publication No. 2008-50695, if the Ag—Sn alloy formed on the outermost surface is exposed to a heating environment, tin oxide is inevitably formed on the outermost surface to cause high resistance. As just described, in any case, heat resistance is insufficient, i.e. a resistance increase caused by being left at high temperature is not suppressed.
Furthermore, since the hard silver plating is softened by being left in the heating environment in the configuration of Japanese Unexamined Patent Publication No. 2009-79250, heat resistance in the sense of preventing softening caused by being left at high temperature also cannot be obtained.
In addition, since a plating layer is produced by a special film forming method in any of methods of cited literatures 1 to 4, production cost increases.
A problem sought to be solved by the present invention is to provide a plated member and a plated terminal for connector, to which a large current can be applied and which have both a low friction coefficient and high heat resistance, at low cost and to provide a method for producing such a plated member and a method for producing such a plated terminal for connector.